1. Field of Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, the present invention relates to a high capacity lithium electrochemical cell designed for high rate discharge applications, such as are required to power a cardiac defibrillator.
2. Prior Art
Early ventricular cardiac defibrillators used two lithium batteries, in series, as their power source. Due to the progress of new circuit designs, the electronic circuit in a defibrillator now consumes less energy than required by earlier models. This makes it possible for the present generation of defibrillators to be powered by a single lithium electrochemical cell. With a one cell design, the requirement for high current pulse capability, or power density, is even greater due to the lowered pulsing voltage. Large electrode surface area is thus needed to accomplish this requirement. In general, when a cell""s electrode surface area is increased, more inert materials (current collector, separator, etc.) are incorporated into the casing. As a result, the cell volumetric capacity is decreased. One of the concerns in such a design is the longevity of the medical device, which is dependent on the cell""s capacity and power efficiency.
The capacity of an electrochemical cell is not only dependent on the electrode design and packing efficiency, it is also dependent on the type of electrode active materials used. For example, for silver vanadium oxide (SVO) cells, the E-phase having the formula AgV2O5.5 is preferred as the cathode active material. Its theoretical volumetric capacity is determined to be 1.37 Ah/ml. By comparison, the theoretical volumetric capacity of fluorinated carbon (CFx) cathode active material (x=1.1) is 2.42 Ah/ml, which is 1.77 times greater than the theoretical capacity of SVO. However, in cardiac defibrillator applications, SVO is preferred because it can deliver relatively high current pulses or high energy within a short period of time. Although the CFx active material has higher volumetric capacity, it cannot be used in such applications due to its relatively low to medium rate of discharge capability.
Attempts to use high capacity materials, such as CFx, by mixing them with a high rate cathode material, such as SVO, are described in U.S. Pat. No. 5,180,642 to Weiss et al. Batteries made from such cathode composites exhibit a relatively lower rate capability in comparison to those having SVO as the sole cathode active material. The benefit of increasing the cell""s theoretical capacity by using CFx as part of the cathode mix is, in part, canceled by the lowering of this material""s relatively high power capability during high rate discharge applications.
Another approach to improving the longevity of cardiac defibrillators powered by cells housed in a single casing is reported in U.S. Pat. No. 5,614,331 to Takeuchi et al. This patent describes a method of using a medium rate cell to power the circuitry of the implantable defibrillator and, separately, an SVO cell as the power supply for the device under high rate applications. The cells are described as being housed within a single casing and activated by the same electrolyte. The advantage of this method is that all of the relatively high power of the SVO cell is reserved for high power pulse discharge applications while the low power requirements of monitoring the heart beat and the like are provided by a relatively high capacity active material, such as CFx. However, this method requires a very careful design to balance the capacities of the high power SVO cell with the low power CFx cell to ensure that both reach end of service life at or near the same time. Such a balance, nevertheless, is very difficult to achieve due to the variable situations in device usage by patients.
The present invention provides for improved discharge performance of lithium electrochemical cells through a new design and a new method of cell discharge. The present invention also provides a new design in defibrillator batteries to improve battery capacity and utilization efficiency, and at the same time to maintain the high current pulse discharge capability throughout service life of the battery. These objectives are achieved by discharging an SVO cell connected in parallel with a CFx cell.
Accordingly, an SVO cell for providing high power capability and a CFx cell for providing high volumetric capacity are connected together in parallel. The anode of the SVO cell is connected to the anode of the CFx cell and the cathode of the SVO cell is connected to the cathode of the CFx cell. The cells are hermetically housed in a single casing activated with the same electrolyte or hermetically housed in separate cases. The present cell configuration is particularly useful in high rate discharge applications, such as required by cardiac defibrillators. In particular, the SVO cell provides high rate discharge while the CFx cell is useful to achieve long service life. Furthermore, end of service life indication during parallel discharge of this novel cell configuration is the same as that of the SVO cell. In other words, both cells reach end of life at the same time in spite of varied usage in actual defibrillator applications. Since both cells reach end of service life at the same time, no discharge energy is wasted and there is no need to balance the capacities of both cells.
At beginning of life, a typical SVO cell has an under load voltage of around 3.2V. In comparison, a typical CFx cell has an under load voltage of around 2.8V. According to the present invention, conservation of the high power energy of the SVO cell and the low power energy of the CFx cell in an implantable cardioverter defibrillator application is achieved by discharging the two cells separately until both reach the same voltage. Then, both cells are discharged in parallel.
These and other aspects of the present invention will become more apparent to those skilled in the art by reference to the following description and to the appended drawings.